A Moon of Saturn Is Habitable

Today Earthlings came one very giant step closer to finding life elsewhere in our solar system. In the final months of its 20-year mission, the spacecraft Cassini delivered its most noteworthy revelation yet: the ocean of Enceladus, a moon of Saturn, is releasing hydrogen, an energy source for some microorganisms. In other words, that ocean is inhabitable. “Enceladus,” says Cornell University astrophysicist Jonathan Lunine, “is the place to go to look for life.”

The ocean—made of liquid water and resembling a hybrid of the Atlantic Ocean, a desert mineral lake and the fluid found near hydrothermal vents—covers the entire surface of this moon. A thick shell of ice surrounds the entire body of water, though, leaving it dark and frigid. But something happening inside that ocean is strong enough to break through those miles of ice. At the moon’s southern pole, a geyser-like plume spews water vapor, ice, salt and a mix of gases hundreds of miles into space at a force of 800 miles per hour.

Enceladus, the sixth-largest moon of Saturn, in an image obtained by the ‘Cassini’ spacecraft in July 2015. In a new study, NASA scientists report that Enceladus is habitable.JPL CALTECH/SPACE SCIENCE INSTITUTE/NASA/REUTERS

Among the contents of the plume, one gas stands out: hydrogen. Although this element is abundant in the universe, it’s usually bound to others, like oxygen. Rarely is hydrogen found alone. But the mere presence of pure hydrogen gas is not why scientists think Enceladus is habitable; it’s the large amount of it. When Cassini flew past the plume in 2015, it detected pure hydrogen in quantities that could not be easily explained.

Hydrogen is the lightest element, readily escaping the gravitational pull of whatever planet or moon it came from. “Molecular hydrogen doesn’t hang around for a long time,” says Lunine. Because this nearly weightless gas floats away so easily, maintaining large amounts requires continual production. The hydrogen on Enceladus is not a relic of an old geological process. Something on that moon is making pure hydrogen gas now. “That’s the exciting part,” says Lunine.